Lithographic printing plates are commonly used in the graphic arts. Most lithographic printing plates comprise a metal substrate coated with a layer of radiation sensitive composition. To form a printing plate, the printer image-wise exposes the radiation sensitive layer to activating wavelengths of actinic radiation. Depending on the nature of the radiation sensitive composition, this exposure will either promote polymerization or curing of the radiation sensitive composition (a negative system) or will cause degradation of the radiation sensitive composition (a positive system). In a negative system, the printer ordinarily develops the image-wise exposed plate by removing the unexposed portions of the radiation sensitive layer.
The radiation sensitive layers in negative systems frequently comprise a diazonium compound and a polymeric binder. Such radiation sensitive layers are typically either "aqueous" developable or "solvent" developable. Solvent developable printing plates typically have good durability and solvent resistance because they use tough polymeric binders. Unfortunately, the use and disposal of the solvent developer creates safety and environmental concerns. Aqueous developable printing plates are much more acceptable with regard to safety and environmental issues, but tend to be less durable and resistant to solvents. The choice of binder is known to affect the durability and solvent resistance of aqueous developable printing plates.
Several U.S. Patents teach the use of specific acetal polymers as the binder for the radiation sensitive layer of a lithographic printing plate. U.S. Pat. No. 3,847,614 discloses an acetal polymer having an aromatic substituted acetal group and a carboxylic acid group.
U.S. Pat. No. 4,652,604 teaches the use of polyacetals comprising six membered cyclic acetals, five membered cyclic acetals and intermolecular acetals. U.S. Pat. No. 4,741,985 teaches the use of an acetal polymer having a six membered cyclic acetal group. The acetal polymers of U.S. Pat. No. 4,940,646 comprise six membered cyclic acetal units derived from an aldehyde that contains hydroxyl groups.
U.S. Pat. Nos. 5,219,699 and 5,169,898 disclose the use of acid substituted ternary acetal polymers. These polymers comprise six membered cyclic acetal repeating groups of three types: (1) unsubstituted or substituted with an alkyl or hydroxyalkyl group, (2) substituted with an aromatic or an heterocyclic moiety, and (3) substituted with a group containing an acid moiety.
U.S. Pat. No. 5,169,897 discloses the use of binary acetal polymers comprising (1) an unsubstituted or alkyl or hydroxyl alkyl substituted, six-membered cyclic acetal and (2) an aromatic or heterocyclic substituted, six-membered cyclic acetal.
All of the above polymers require the presence of aliphatic substituted or non-substituted cyclic acetals. In fact, U.S. Pat. Nos. 5,219,699 and 5,169,898 indicate that if the alkyl or hydroxyalkyl substituted cyclic acetal is missing the performance of plates made with the polymer deteriorates.
While lithographic printing plates using the above mentioned acetal polymers generally perform in an effective manner, improvements in solvent resistance and press durability remains a critical need in the art.